The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to an image forming apparatus of the type sequentially transferring toner images from an image carrier to a belt or similar intermediate image transfer body one upon the other (primary transfer) and collectively transferring them from the intermediate transfer body to a paper or similar recording medium (secondary transfer), and the intermediate transfer body.
Today, in the electrophotographic imaging art, image transfer using a belt or a roller is replacing traditional corona image transfer from the environmental standpoint. The present invention relates to image transfer using an image transfer roller.
Various kinds of technologies have heretofore been proposed in relation to an image transfer roller. For example, an image transfer roller may be implemented by a sponge roller having a specific hardness for reducing a nip pressure. This transfer roller is directed toward an image free from vermicular defects. The surface of an image transfer roller may have its specific resistance increased in order to prevent an image transfer voltage from varying with the varying environment, e.g., the resistance of a recording medium, as taught in Japanese Patent Laid-Open Publication No. 2-212873.
At the time of secondary transfer following primary transfer, an optimal image transfer current to be applied to the image transfer roller is determined by the transfer characteristic of a sol id portion and that of a halftone portion. That is, a current satisfying both of such characteristics has customarily been selected as an optimal image transfer current. In a low temperature, low humidity environment, it is a common practice to increase a bias to be applied to the roller also because the resistance of a recording medium increases in the above environment.
However, the conventional image transfer roller has the following problems when used in the above conditions. Desirable image transfer is not achievable because the range of optimal currents is extremely narrow or does not exist at all, i.e., the transfer characteristic of a solid portion and that of a halftone portion cannot be implemented at the same time. Further, because the resistance of the roller susceptible to the environment, it is necessary to use a power pack having an extremely great capacity or to heat the roller or otherwise prevent the resistance of the roller from increasing.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-19276, 8-87156, 8-234564, 10-78713, 10-228187, and 11-38782.